<h2>DESCRIPTION</h2>

<em>r.surf.area</em> calculates area of regular 3D triangulated points
(centers of cells) in current region by adding areas of triangles.
Therefore, area of a flat surface will be reported as (<tt>rows + cols
-1) * (area of cell)</tt> less than area of flat region due to a half
row and half column missing around the perimeter.

<h2>NOTES</h2>

This calculation is heavily dependent on data resolution (think of it
as a fractal shoreline problem, the more resolution the more detail,
the more area, etc). This module uses the <b>current region
settings</b>, not the resolution of the raster map. This is especially
important for surfaces with <tt>NULL</tt> values and highly irregular
edges. The module does not [currently] attempt to correct for the
error introduced by this <em>edge effect</em>.

<p>This version actually calculates area twice for each triangle pair,
keeping a running minimum and maximum area depending on the direction
of the diagonal used.

<p>Reported totals are:
<ol>
<li>"Plan" area of <tt>NULL</tt> values within the current GRASS
region</li>
<li>"Plan" area within calculation region (<tt>rows-1 * cols-1 *
cellarea</tt>)</li>
<li>Average of the minimum and maximum calculated 3d triangle area
within this region</li>
<li>"Plan" area within current computational region (<tt>rows * cols *
cellarea</tt>)</li>
<li>Scaling of calculated area to current region</li>
</ol> 

<p><em>r.surf.area</em> works best when the surface being evaluated
extends to the edges of the current region and the cell resolution is
small. Surfaces which are especially long and thin and have highly
irregular boundaries will tend to have underestimated surface areas.
Setting a high cell resolution (small area) will greatly reduce this
impact, but will cause longer processing times.

<h2>EXAMPLES</h2>

<div class="code"><pre>
g.region -p raster=elevation

r.surf.area map=elevation units=hectares
Null value area ignored in calculation: 0.000000
Plan area used in calculation: 20221.510000
Surface area calculation(low, high, avg):
        20294.310421 20320.936368 20307.623395
Current region plan area: 20250.000000
Estimated region Surface Area: 20336.234719
</pre></div>

<h2>SEE ALSO</h2>

<em>
<a href="g.region.html">g.region</a>,
<a href="r.surf.idw.html">r.surf.idw</a>,
<a href="r.surf.fractal.html">r.surf.fractal</a>,
<a href="r.surf.gauss.html">r.surf.gauss</a>,
<a href="r.volume.html">r.volume</a>,
<a href="r.slope.aspect.html">r.slope.aspect</a>,
<a href="v.to.rast.html">v.to.rast</a>
</em>

<h2>AUTHOR</h2>
Bill Brown, USACERL December 21, 1994
<br>
Modified for floating point rasters and <tt>NULL</tt> values by Eric
G. Miller (October 17, 2000)
<br>
Updated for GRASS 7, and units option by Martin Landa, Czech Technical
University in Prague, Czech Republic (October 2011)

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